1. Field of the Invention
The invention relates to a liquid crystal composition for use in an AM (active matrix) device, and an AM device containing the composition. The composition has a nematic phase and a positive dielectric anisotropy.
2. Related Art
In liquid crystal display devices, classification is based on an operating mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), and so forth. Classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and so forth, and AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. Classification based on a light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight, and a semi-transmission type utilizing both natural light and a backlight.
These devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes the relationship between the general characteristics of the composition and the AM device. The general characteristics of the composition will be explained further based on a commercially available AM device. The temperature range of a nematic phase relates to the temperature range, in which the device can be used. The desirable maximum temperature of a nematic phase is 70° C. or more, and a desirable minimum temperature of a nematic phase is −10° C. or less. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying a moving image. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of Liquid Crystal Composition and AM DeviceGeneral CharacteristicsGeneral CharacteristicsNoof a Compositionof an AM Device1Temperature range of a nematicUsable temperaturephase is widerange is wide2Viscosity is small1Response time is short3Optical anisotropy is suitableContrast ratio is large4Dielectric anisotropy is largeThreshold voltage is low andelectric powerconsumption is smalland a contrast ratio is large5Specific resistance is largeVoltage holding ratio is largeand a contrast ratio is large6Composition is stable to ultravioletService life is longray and heat1A liquid crystal composition can be injected into a cell in a short time.
The optical anisotropy of the composition relates to the contrast ratio of the device. A product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed to maximize the contrast ratio. A suitable value of the product depends on the kind of operation mode. In a device having a TN mode and so forth, a suitable value is approximately 0.45 μm. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio. Accordingly, a large dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at room temperature and also at a high temperature in the initial stage. A composition having a large specific resistance is desirable at room temperature and also at a high temperature after it has been used for a long time. A stability of the composition to an ultraviolet ray and heat relates to a service life of the liquid crystal display device. In the case where the stability is high, the device has a long service life. These characteristics are desirable for an AM device used in a liquid crystal projector, a liquid crystal television and so forth.
Conventional compositions are disclosed in the following patent documents: JP H6-504032 A/1994 (WO 91/08184 A), GB 2,257,701 A, JP 2003-301178 A (US 2003/0222244 A), WO 2004/035710 A and WO 2005/007775 A.
Desirable AM device characteristics include: a usable temperature range is wide, a response time is short, a contrast ratio is large, a threshold voltage is low, a voltage holding ratio is large, a service life is long, and so forth. Even one millisecond shorter response time is desirable. Thus, the composition having characteristics such as a maximum temperature of a nematic phase, a minimum temperature of a nematic phase, a small viscosity, a large optical anisotropy, a large dielectric anisotropy, a large specific resistance, a high stability to an ultraviolet ray, a high stability to heat, and so forth is especially desirable.